Cognitive dissonance is a psychological state that describes the uncomfortable feeling between what one holds to be true and what one knows to be true. Similar to ambivalence, the term cognitive dissonance describes conflicting thoughts or beliefs (cognitions) that occur at the same time, or when engaged in behaviors that conflict with one's beliefs. In academic literature, the term refers to attempts to reduce the discomfort of conflicting thoughts, by performing actions that are opposite to one's beliefs.

In simple terms, it can be the filtering of information that conflicts with what one already believes, in an effort to ignore that information and reinforce one's beliefs. In detailed terms, it is the perception of incompatibility between two cognitions, where "cognition" is defined as any element of knowledge, including attitude, emotion, belief, or behavior.
The theory of cognitive dissonance states that contradicting cognitions serve as a driving force that compels the mind to acquire or invent new thoughts or beliefs, or to modify existing beliefs, so as to reduce the amount of dissonance (conflict) between cognitions. Experiments have attempted to quantify this hypothetical drive. Some of these have examined how beliefs often change to match behavior when beliefs and behavior are in conflict.

Social psychologistLeon Festinger first proposed the theory in 1957 after the publication of his book When Prophecy Fails, observing the counterintuitive belief persistence of members of a UFOdoomsday cult and their increased proselytization after the leader's prophecy failed. The failed message of earth's destruction, purportedly sent by aliens to a woman in 1956, became a disconfirmed expectancy that increased dissonance between cognitions, thereby causing most members of the impromptu cult to lessen the dissonance by accepting a new prophecy: that the aliens had instead spared the planet for their sake.

Maintaining conflicting principles (e.g. logically incompatible beliefs) or rejecting reasonable behavior to avoid conflict can be increasingly maladaptive (non-beneficial) as the gap being bridged widens, and popular usage tends to stress the maladaptive aspect. Cognitive dissonance is often associated with the tendency for people to resist information that they don't want to think about, because if they did it would create cognitive dissonance, and perhaps require them to act in ways that depart from their comfortable habits. They usually have at least partial awareness of the information, without having moved to full acceptance of it, and are thus in a state of denial about it. This "irrational inability to incorporate rational information" is perhaps the most common perception of cognitive dissonance, and this or another example of extreme maladaption would appear to be underlying many conceptions of the term in popular usage.

Studies have not so far detected any gender or cross-cultural differences.